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><A
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>44.1. The Path of a Query</A
></H1
><P
>    Here we give a short overview of the stages a query has to pass in
    order to obtain a result.
   </P
><DIV
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><P
>      A connection from an application program to the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
      server has to be established. The application program transmits a
      query to the server and waits to receive the results sent back by the
      server.
     </P
></LI
><LI
CLASS="STEP"
><P
>      The <I
CLASS="FIRSTTERM"
>parser stage</I
> checks the query
      transmitted by the application
      program for correct syntax and creates
      a <I
CLASS="FIRSTTERM"
>query tree</I
>.
     </P
></LI
><LI
CLASS="STEP"
><P
>      The <I
CLASS="FIRSTTERM"
>rewrite system</I
> takes
      the query tree created by the parser stage and looks for
      any <I
CLASS="FIRSTTERM"
>rules</I
> (stored in the
      <I
CLASS="FIRSTTERM"
>system catalogs</I
>) to apply to
      the query tree.  It performs the
      transformations given in the <I
CLASS="FIRSTTERM"
>rule bodies</I
>.
     </P
><P
>      One application of the rewrite system is in the realization of
      <I
CLASS="FIRSTTERM"
>views</I
>.
      Whenever a query against a view
      (i.e., a <I
CLASS="FIRSTTERM"
>virtual table</I
>) is made,
      the rewrite system rewrites the user's query to
      a query that accesses the <I
CLASS="FIRSTTERM"
>base tables</I
> given in
      the <I
CLASS="FIRSTTERM"
>view definition</I
> instead.
     </P
></LI
><LI
CLASS="STEP"
><P
>      The <I
CLASS="FIRSTTERM"
>planner/optimizer</I
> takes
      the (rewritten) query tree and creates a
      <I
CLASS="FIRSTTERM"
>query plan</I
> that will be the input to the
      <I
CLASS="FIRSTTERM"
>executor</I
>.
     </P
><P
>      It does so by first creating all possible <I
CLASS="FIRSTTERM"
>paths</I
>
      leading to the same result. For example if there is an index on a
      relation to be scanned, there are two paths for the
      scan. One possibility is a simple sequential scan and the other
      possibility is to use the index. Next the cost for the execution of
      each path is estimated and the cheapest path is chosen.  The cheapest
      path is expanded into a complete plan that the executor can use.
     </P
></LI
><LI
CLASS="STEP"
><P
>      The executor recursively steps through
      the <I
CLASS="FIRSTTERM"
>plan tree</I
> and
      retrieves rows in the way represented by the plan.
      The executor makes use of the
      <I
CLASS="FIRSTTERM"
>storage system</I
> while scanning
      relations, performs <I
CLASS="FIRSTTERM"
>sorts</I
> and <I
CLASS="FIRSTTERM"
>joins</I
>,
      evaluates <I
CLASS="FIRSTTERM"
>qualifications</I
> and finally hands back the rows derived.
     </P
></LI
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><P
>    In the following sections we will cover each of the above listed items
    in more detail to give a better understanding of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s internal
    control and data structures.
   </P
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